As shown in FIG. 1, an optical disc 11, such as a compact disc (CD) or a digital versatile disc (DVD), is generally divided into three areas: a lead-out area, a data area, and a lead-in area. The lead-out area includes information indicating an end track. The data area includes audio information, video information, and so forth. The lead-in area includes a list of the information recorded in the data area, i.e., a table of contents (TOC) including information such as addresses of start and end tracks, and other various types of information related to the optical disc 11.
A micro controller unit (MCU) of a conventional optical disc system controls movement of an optical pickup to an innermost perimeter of the optical disc 11 to read the TOC information when the optical disc 11 is loaded into the conventional optical disc system. The conventional optical disc system also uses a limit switch to control movement of the optical pickup to the innermost perimeter of the optical disc 11.
FIG. 2 is a block diagram of a conventional optical disc system including a limit switch. Referring to FIG. 2, an optical disc system 10 includes an optical pickup 12, a radio frequency (RF) amplifier 13, a digital signal processor (DSP) 14, a servo signal processor (SSP) 15, a MCU 16, a digital-to-analog converter (DAC) 17, a servo driver 18, a spindle motor 23, a sled motor 24, and a limit switch 25. The servo driver 18 includes a focus servo driver 19, a tracking servo driver 20, a sled servo driver 21, and a spindle servo driver 22. The limit switch 25 is positioned in the vicinity of the innermost perimeter of the optical disc 11. One node of the limit switch 25 is grounded, and the other node of the limit switch 25 is connected to the MCU 16.
FIG. 3 is a timing diagram showing major signals used to move the optical pickup 12 of the optical disc system 10 of FIG. 2 to the innermost perimeter of the optical disc 11. As shown in FIG. 3, the limit switch 25 outputs a limit signal LMS with a predetermined voltage level when the limit switch 25 is switched off. The limit switch 25 outputs the limit signal LMS with a ground voltage level when the limit switch 25 is switched on.
The limit switch 25 is switched on when the sled motor 24 moves the optical pickup 12 to the innermost perimeter of the optical disc 11, and thus the optical pickup 12 is connected to the limit switch 25. As a result, the limit switch 25 outputs the limit signal LMS with the ground voltage level to the MCU 16. When the MCU 16 receives the limit signal LMS with the ground voltage level, the MCU 16 determines that the optical pickup 12 has moved to the innermost perimeter of the optical disc 11 and outputs a predetermined control signal to the SSP 15 so as to stop operation of the sled motor 24. Here, the sled motor 24 is controlled by a voltage level of a control signal SLD output from the sled servo driver 21. In more detail, as shown in FIG. 3, when the control signal SLD has a less voltage level than a predetermined reference voltage level, the sled motor 24 rotates in a reverse direction. When the control signal SLD has the same voltage level as the predetermined reference voltage level, the sled motor 23 stops its rotation operation.
Meanwhile, to make optical disc systems lighter and slimmer, various methods have been investigated to control the movement of an optical pickup to an innermost perimeter of an optical disc without using a limit switch. For example, one method is to control the movement of the optical pickup to the innermost perimeter of the optical disc using sub-Q data recorded on the optical disc. Sub-Q data refers to information recorded in a data area of the optical disc and includes, for example, information on time required for reproducing the recorded data. In this method, sub-Q data is read at predetermined intervals in the data area while the optical pickup moves toward the innermost perimeter of the optical disc, a current position of the optical pickup is calculated from the read sub-Q data, and the optical pickup is moved to the innermost perimeter of the optical disc. The problem with this method is that a focus servo, a tracking servo, a spindle servo, and a sled servo must be driven to actuate the moving parts of the optical disc system, and the sub-Q data must be iteratively read. Thus, it takes a long time to read TOC information.
Another example of a method of controlling movement of an optical pickup to the innermost perimeter of an optical disc without using a limit switch is disclosed in U.S. Pat. No. 5,173,887. In this method, the distance the optical pickup must move to get to the innermost perimeter of the optical disc is determined experimentally for various starting positions and put into a table. During operation of the optical pickup, the starting position is determined by reading sub-Q data from the optical disc. Then, the distance to the innermost perimeter of the optical disc is obtained from the table, and the optical pickup is moved the obtained distance toward the innermost perimeter of the optical disc. The problem with this method is that the distances stored in the table are imprecise and sometimes inaccurate. Also, additional memory is required to store the table.
Therefore, a need exists for an optical disc system and a method for reducing the time required for reading TOC information and accurately controlling the movement of an optical pickup to an inner perimeter of an optical disc.